This invention relates to apparatus for detecting the level of a particulate material in a vessel and more particularly it is concerned with an apparatus for detecting the level of toner particles contained in a toner supply tank.
In electrophotographic copying apparatus in which developing is carried out by using a magnetic brush, the developing agent used consists of iron powder and toner adhered thereto and a magnetic brush is formed by the magnetic force of permanent magnets. The magnetic brush is brought into contact with an electrostatic latent image and causes the toner to adhere to the latent image to convert the same to a visible image. The concentration of the toner in the developing agent is gradually lowered as the cycle of developing operations is repeated, and thus it becomes necessary to detect the lowering of the concentration of the toner for supplying toner to the developing tank to replenish the toner therein. The toner to be supplied to the developing tank is stored in the toner supply tank beforehand.
If the toner supply tank runs out of toner and no toner is supplied to the developing agent tank, the toner concentration of the developing agent will be lowered and trouble will be caused in the developing operation. This makes it necessary to provide means whereby the level of the toner in the toner supply tank is detected and to give indication showing that the quantity of the toner in the toner supply tank has dropped below a predetermined level. In one method known in the art to detect the level of the toner in the toner supply tank, light is caused to be incident on the toner supply tank which is made of a transparent material or synthetic resin and the shadow of the toner formed at one end of the light is detected. In another method known in the art, a detecting member provided with blades is mounted in a toner supply tank which is rotated and the quantity of the toner is detected by the force of attraction of magnets and the resistance offered by the toner to rotation. These methods of the prior art have disadvantages in that the mechanisms for carrying the methods into practice and the control system therefor are complex in construction.